I want to add a small sub to my car. I would like to kill all of the bass signal to my current speakers. It is what killed them in the first place.

I have a tweeter at the top of each door, a 6.5" speaker in the bottom of the door and a set of 6x9s in the rear.

I would like the tweeter to be the highs, the 6.5 in the door mid and the 6x9 a high/mid mix.

I would then add a small sub box for bass.

Any ideas how to get this setup going?

 

Simple, Either an amp for the interior that will block all the bass using only high pass, but you will need a cross over for the fronts, so the tweeters don't get any kind of bass.

OR

Using bass blockers on the rear and a cross over for the fronts and use the stereo amp (but they suck though, but you'll get some decent sound)

You will definitely need some crossovers for the front. But other than that, there is a world of possibilities. I would get an amp for the interiors, a bass amp with sub and an EQ. This way you can fine tune your sound instead of just treble and bass.